JP2005286778A - filter - Google Patents

Abstract

A filter having a large attenuation using a gmC filter can be formed on a MOS integrated circuit substrate.
A filter 11 includes a plurality of transconductance amplifiers gm1, gm2,... And capacitors C1, C2,..., A gmC filter 13 having a characteristic of passing a signal in a specific band, and a high band in a stop band. The LPF 12 has a characteristic of attenuating the signal on the side, and the HPF 14 has a characteristic of attenuating the signal on the low band side of the stop band. By configuring the filter 11 as described above, a bandpass filter having a large stopband attenuation can be formed on the MOS integrated circuit substrate.
[Selection] Figure 1

Description

  The present invention relates to a filter formed on a MOS integrated circuit.

  In order to integrate the circuit of the wireless receiver, it has been considered that a filter attached to the outside of an IC (integrated circuit) is conventionally formed of an active filter and formed on the IC. As an active filter, a gmC filter or the like is known. The following is a conventional technique of the gmC filter.

In Patent Document 1, a band attenuation filter of a video signal of a television is configured by two transconductance circuits having variable current sources, thereby reducing the variable range of the current of the control current source and widening the cutoff frequency range. It is described.
Japanese Patent Application Laid-Open No. H10-228688 discloses a third and fourth transconductance amplifier in which an active bandpass filter is composed of first and second transconductance amplifiers and an active band attenuation filter is the same as the first and second transconductance amplifiers. And realizing an active band attenuating filter that is not affected by the gain characteristics of the element.

Patent Document 3 describes that a low-pass filter and a high-pass filter constituted by a biquad circuit are formed on a semiconductor integrated circuit to realize a band-pass filter having flat frequency characteristics.
JP-A-8-330901 Japanese Patent Laid-Open No. 11-163677 JP-A-8-172338

  The gmC filter can realize a filter having a desired frequency characteristic by changing the mutual conductance gm. However, as shown in FIG. 3, the gmC filter is limited by noise generated in the transistor, and has a drawback that the attenuation does not increase beyond a certain value.

In the field of communication equipment, for example, radio receivers, IF filters that extract IF signals (intermediate frequency signals) placed after the mixer circuit should increase the attenuation of the stopband to increase the noise margin. Is desired.
An object of the present invention is to enable a filter with a large attenuation using a gmC filter to be formed on a MOS integrated circuit substrate.

  The filter of the present invention attenuates a gmC filter having a characteristic of allowing a signal having a frequency in a specific band to pass and attenuating a signal having a frequency outside the specific band, and a signal having a frequency on the lower side of the stop band of the gmC filter. A high-pass filter composed of a passive element having characteristics and a low-pass filter composed of a passive element having a characteristic of attenuating a signal having a frequency higher than the stop band of the gmC filter were formed on a MOS integrated circuit substrate.

According to the present invention, since the passband frequency is determined by the gmC filter and the stopband signal is attenuated by the high-pass filter and the low-pass filter made of passive elements, the bandpass filter having high frequency accuracy and a large attenuation is obtained. It can be formed on a MOS integrated circuit substrate.
According to another aspect of the present invention, in the above invention, the gmC filter is detected by a differential amplifier circuit, a common-mode voltage detection circuit that detects a common-mode voltage of the differential amplifier circuit, and the common-mode voltage detection circuit. A transconductance amplifier having a bias control circuit that controls the bias of the differential amplifier circuit so that the common-mode voltage of the differential amplifier circuit approaches a reference value based on the common-mode voltage and a predetermined reference value.

With this configuration, it is possible to suppress the fluctuation of the DC potential of the output of the mutual conductance amplifier and improve the characteristics of the gmC filter.
The above-mentioned common-mode voltage detection circuit corresponds to, for example, the buffer circuit 23, resistors R1 and R2, and the source follower 24 shown in FIG. 2, and the bias control circuit includes the comparison circuit 25 and MOS transistors Q3 and Q4 shown in FIG. Corresponding to

  According to another aspect of the present invention, the gmC filter includes a first MOS transistor and a second MOS transistor to which a signal having a phase difference of 180 ° is input, and an output voltage of the first and second MOS transistors at a gate. A third MOS transistor and a fourth MOS transistor having at least two resistors connected in series between their sources, a voltage corresponding to the midpoint voltage of the two resistors, and a predetermined reference value. A transconductance amplifier including a comparison circuit for comparison and a bias control circuit for controlling the common-mode output voltages of the first and second MOS transistors to approach a reference value based on a comparison result of the comparison circuit;

With this configuration, it is possible to suppress the fluctuation of the DC potential of the output of the mutual conductance amplifier and improve the characteristics of the gmC filter.
The above comparison circuit corresponds to, for example, the comparison circuit 25 in FIG. 2, and the bias control circuit corresponds to the MOS transistors Q3 and Q4 in FIG.

  Another filter of the present invention includes a gmC file having a characteristic of passing a signal having a frequency higher than a specific frequency and attenuating a signal having a frequency lower than the specific frequency, and a passive element having a characteristic of attenuating a signal having a frequency lower than the specific frequency. A high-pass filter made of is formed on a MOS integrated circuit substrate.

According to the present invention, a high-pass filter having a large stopband attenuation and high frequency accuracy can be formed on a MOS integrated circuit substrate.
Another filter of the present invention includes a gmC file having a characteristic of passing a signal having a frequency lower than a specific frequency and attenuating a signal having a frequency higher than the specific frequency, and a passive element having a characteristic of attenuating a signal having a frequency higher than the specific frequency. A low-pass filter consisting of the following was formed on a MOS integrated circuit substrate.

  According to the present invention, a low-pass filter having a large stopband attenuation and high frequency accuracy can be formed on a MOS integrated circuit substrate.

  According to the present invention, by combining the gmC filter and the passive filter, a filter with high frequency accuracy and a large attenuation amount of the signal in the stop band can be formed on the MOS integrated circuit substrate.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating a configuration of a filter according to the embodiment.
The gmC filter 13, the low-pass filter 12, and the high-pass filter 14 described below are semiconductor integrated circuits manufactured by a CMOS process capable of forming p-channel MOS transistors and n-channel MOS transistors, for example, semiconductor integrated circuits for FM and AM radio receivers. Mounted on the board.

In FIG. 1, an IF filter 11 according to the embodiment is a filter disposed at a subsequent stage of a mixer circuit such as a radio receiver, and has a characteristic of passing an IF signal and blocking signals of other frequencies.
The IF filter 11 includes a gmC filter 13, a low-pass filter (hereinafter referred to as LPF) 12 and a high-pass filter (hereinafter referred to as HPF) 14 connected to the subsequent stage.

  The LPF 12 is a filter composed of a passive element of a resistor and a capacitor, and has a specific frequency (for example, a frequency that is higher than the upper limit frequency of the pass band of the gmC filter 13 determined in consideration of an error between the resistance value and the capacitance) or less. It has a characteristic of allowing a signal to pass through and attenuating a signal having a frequency higher than the specific frequency (the high frequency side of the stop band).

  The gmC filter 13 includes a plurality of transconductance amplifiers gm1, gm2,... and capacitors C1, C2,..., and has a characteristic of allowing a signal in a specific band to pass and attenuating a signal outside the band. Since the gmC filter 13 can change the frequency characteristics of the filter by changing the mutual conductance gm of the conductance amplifiers gm1, gm2,..., the gmC filter 13 is centered on the design frequency (frequency of the target IF signal). A bandpass filter having passband characteristics and high frequency accuracy can be realized.

  The HPF 14 is a filter composed of a passive element of a resistor and a capacitor, and has a specific frequency (for example, a frequency lower than a lower limit frequency of the pass band of the gmC filter 13 determined in consideration of an error in resistance value and capacitance) or more. It has a characteristic of allowing a signal having a frequency to pass therethrough and attenuating a signal having a frequency lower than the specific frequency (lower side of the stop band).

By configuring the IF filter 11 as described above, the target IF signal is selected by the gmC filter 13 with high frequency accuracy, and the signal in the stop band is attenuated by the gmC filter, the HPF 14 and the LPF 12, and therefore other than the IF signal. Can be sufficiently attenuated.
FIG. 2 is a diagram illustrating a configuration example of the transconductance amplifier according to the present embodiment.

The conductance amplifier shown in FIG. 1 includes a differential amplifier circuit 21 composed of MOS transistors Q1 to Q4 and a current source I1, and a CMFB (Common Mode Feed Back) circuit for controlling the bias to the differential amplifier circuit 21. 22.
The CMFB circuit 22 includes resistors R1 and R2 connected to the output terminal of the differential amplifier circuit 21 through a buffer circuit 23 composed of MOS transistors Q5 and Q6, and an output from the middle point of the resistors R1 and R2 and a reference. A comparison circuit composed of a source follower 24 composed of MOS transistors Q8 and Q9 that receive the output of the MOS transistor Q7 corresponding to the voltage Vref, and a MOS transistor Q10 to Q12 that receives the output from the source follower 24. 25, and a phase compensation circuit (capacitor C1) for performing phase compensation by connecting to the output terminal from the comparison circuit 25 and the gate of the MOS transistor Q9.

In the differential amplifier circuit 21, when the input voltages Vin and Vip are applied to the gates of the MOS transistors Q1 and Q2, voltages corresponding to the applied voltages are output to the output terminals as the output voltages Vop and Von.
The output voltages Vop and Von are respectively input to the resistors R1 and R2 through the buffer circuit 23 composed of the MOS transistors Q5 and Q6, and the midpoint voltage Vm is taken out. Since the resistance value of the resistor R1 and the resistance value of the resistor R2 are set to the same value, by taking an output signal from the middle point between the resistors R1 and R2, the source voltage of the MOS transistor Q5 and the source voltage of Q6 are A midpoint voltage Vm is obtained.

  The midpoint voltage Vm and the reference voltage Vref input through the buffer circuit 23 are compensated for by the buffer circuit 23 via the source follower 24, and then the MOS transistor Q10 and the MOS transistor Q10 constituting the comparison circuit 25, respectively. Input to the gate of Q11.

  Here, since the MOS transistors Q3, Q4, and Q12 are current sources configured by current mirror circuits, the voltage change according to the comparison result by the comparison circuit 25 is applied to the MOS transistors Q3 and Q4 via the MOS transistor Q12. Communicated. Therefore, since the gate voltages of the MOS transistors Q3 and Q4 change, the current flowing through the MOS transistors Q1 and Q2 changes, and the DC bias voltage of the conductance amplifier is adjusted to be the same as the reference voltage Vref.

  For example, when the potentials of the output voltages Vop and Von increase, the midpoint voltage Vm of the resistors R1 and R2 connected to the output terminal of the differential amplifier circuit 21 via the buffer circuit 23 also increases. Since the gate voltage of the MOS transistor Q9 rises due to the rise of the midpoint voltage Vm, the gate voltage of the MOS transistor Q11 constituting the comparison circuit 25 rises, and the current between the drain and source of Q11 increases.

  Since the MOS transistors Q10 and Q11 constituting the comparison circuit 25 are connected to the current source on the source side, when the current between the drain and source of the MOS transistor Q11 increases, the current between the drain and source of the MOS transistor is correspondingly increased. The current decreases and the drain voltage of MOS transistor Q12 increases. As the drain voltage of the MOS transistor Q12 rises, the gate voltages of the MOS transistors Q3 and Q4 constituting the differential amplifier circuit 21 rise, so that the source-drain resistance (voltage drop) increases. As a result, the output voltage of the differential amplifier circuit 21 decreases, and the midpoint voltage Vm and the reference voltage Vref are adjusted to be the same.

  As described above, the DC bias voltage of the output voltage from the differential amplifier circuit 21 is compared with the reference voltage Vref input to the gate of the MOS transistor Q7 by the comparison circuit 25, and the DC bias voltage is changed according to the fluctuation of the DC bias. Since the feedback is fed back to the differential amplifier circuit 21 so as to cancel the fluctuation, the DC bias voltage becomes constant.

  Here, since the constant current source of the differential amplifier circuit 21 composed of the MOS transistors Q3 and Q4 is generally high impedance, the output terminal of the differential amplifier circuit 21 and the resistors R1 and R2 do not pass through the buffer circuit 23. And the output gain of the conductance amplifier decreases.

  In general, an operational amplifier including the differential amplifier circuit 21 is ideally designed to have an infinite output-side impedance. However, if the resistors R1 and R2 are directly connected to the output terminal of the differential amplifier circuit 21, the middle point between the resistor R1 and the resistor R2 is used. Is a circuit node that is feedback-controlled so as to have a predetermined reference voltage and does not change in an alternating manner, and a circuit node that does not change in an alternating manner is considered to be equivalent to being connected to a power supply or GND, so that the resistor R1 and the resistor R2 Can be regarded as equivalent to the power supply voltage Vdd, which is equivalent to the MOS transistor Q3 and the resistor R1, and the MOS transistor Q4 and the resistor R2 being connected in parallel. Therefore, the output side impedance is lowered and the output gain is lowered. Further, when the BPF is configured by the conductance amplifier having the configuration in which the output side of the differential amplifier circuit 21 and the resistors R1 and R2 are connected without using the buffer circuit 23 as described above, the output impedance of the conductance amplifier is lowered. In addition, since the output gain decreases, there arises a problem that the gain in the frequency pass band of the BPF decreases.

  Therefore, in the conductance amplifier according to the present embodiment, resistors R1 and R2 are connected to the output side of the differential amplifier circuit 21 via the buffer circuit 23 as shown in FIG. With such a configuration, it is possible to prevent the output side impedance from being lowered and to prevent the output gain from being lowered. Therefore, when the BPF is configured using the conductance amplifier according to the present embodiment, it is possible to prevent the gain of the BPF frequency passband from being lowered.

  Furthermore, in the conductance amplifier according to the present embodiment, the gate of the MOS transistor Q9, which is the input terminal of the source follower 4, and the drain of the MOS transistor Q10, which is the output terminal of the comparison circuit 25, are connected via the capacitor C1. . By feeding back the drain voltage of the MOS transistor Q10, which is the output terminal of the comparison circuit 25, to the gate voltage of the MOS transistor Q9, which is the input terminal of the source follower 24, the difference input by the CMFB and the output terminal of the differential amplifier circuit 21 It is possible to efficiently compensate for the phase between the MOS transistor constituting the current source of the dynamic amplifier circuit 21. Therefore, the conductance amplifier according to this embodiment can be stably operated without oscillating. The gm of the conductance amplifier can be adjusted by changing the currents of the current sources I1 to I5.

  According to the conductance amplifier with CMFB of the above-described embodiment, fluctuations in the DC potential of the output voltage of the differential amplifier circuit 21 can be suppressed. As a result, it is possible to realize a gmC filter having the filter characteristics as designed by stabilizing the operations of the conductance amplifiers gm1, gm2, and the like.

The present invention is not limited to the embodiment described above, and may be configured as follows.
(1) The present invention is not limited to a BPF using a gmC filter, and a high-pass filter may be configured by combining a gmC filter having a high-pass characteristic and an HPF composed of a passive element of a resistor and a capacitor. A low-pass filter may be configured by combining a gmC filter having a low-pass filter and an LPF composed of passive elements.
(2) The conductance amplifier is not limited to the circuit shown in the embodiment, and may be one using another CMFB circuit.
(3) The present invention can be applied not only to a semiconductor integrated circuit for a radio receiver but also to a MOS integrated circuit of a radio communication device and other devices.

It is a figure which shows the structure of the filter of embodiment. It is a circuit diagram of the conductance amplifier of an embodiment. It is a figure which shows the characteristic of a gmC filter.

Explanation of symbols

11 Filter 12 LPF
13 gmC filter 14 HPF
21 differential amplifier circuit 22 CMFB circuit 23 buffer circuit 24 source follower 25 comparison circuit I1 to I5 current source

Claims (7)

A gmC file having a characteristic of passing a signal of a frequency in a specific band and attenuating a signal of a frequency outside the specific band;
A high-pass filter comprising a passive element having a characteristic of attenuating a signal having a frequency on a low frequency side of the stop band of the gmC filter;
A filter in which a low-pass filter made of a passive element having a characteristic of attenuating a signal having a high frequency side of the stop band of the gmC filter is formed on a MOS integrated circuit substrate. The gmC filter is
A differential amplifier circuit;
A common-mode voltage detection circuit for detecting a common-mode voltage of the differential amplifier circuit;
A bias control circuit that controls the bias of the differential amplifier circuit so that the common-mode voltage of the differential amplifier circuit approaches a reference value based on the common-mode voltage detected by the common-mode voltage detection circuit and a predetermined reference value; The filter according to claim 1, comprising a transconductance amplifier comprising: The gmC filter is
First and second MOS transistors to which signals having a phase difference of 180 ° are input;
Third and fourth MOS transistors in which output voltages of the first and second MOS transistors are input to the gates, and at least two resistors are connected in series between the sources;
A comparison circuit that compares a voltage corresponding to the midpoint voltage of the two resistors with a predetermined reference value;
2. The filter according to claim 1, further comprising a transconductance amplifier including a bias control circuit that controls the common-mode output voltages of the first and second MOS transistors to approach a reference value based on a comparison result of the comparison circuit. A first p-channel MOS transistor in which the voltage at the midpoint of the two resistors is input to the gate; and a second p-channel MOS transistor in which the reference voltage is input to the gate;
4. The filter according to claim 3, wherein the comparison circuit compares the source voltages of the first p-channel MOS transistor and the second p-channel MOS transistor and outputs a voltage corresponding to the comparison result to the bias control circuit.   The bias control circuit includes fifth and sixth MOS transistors connected between the drains of the first and second MOS transistors and a power supply, and the fifth and sixth MOS transistors are controlled by the output voltage of the comparison circuit. 5. The filter according to claim 4, wherein the source-drain voltage of the first and second MOS transistors is controlled so that the common-mode output voltage of the first and second MOS transistors approaches a reference value. A gmC file having a characteristic of passing a signal at a specific frequency or higher and attenuating a signal having a frequency lower than the specific frequency;
A filter in which a high-pass filter composed of a passive element having a characteristic of attenuating a signal having a frequency lower than the specific frequency is formed on a MOS integrated circuit substrate. A gmC file having a characteristic of passing a signal below a specific frequency and attenuating a signal having a frequency higher than the specific frequency;
A filter in which a low-pass filter made of a passive element having a characteristic of attenuating a signal having a frequency higher than the specific frequency is formed on a MOS integrated circuit substrate.

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